1. Field of the Invention
The present invention relates to an additive for cement compositions. More particularly, the present invention relates to an additive for Portland cement swimming pool plaster and to pool Portland cement plaster compositions containing the additive.
2. The Prior Art
Each year over 120,000 swimming pools are plastered. Many of these swimming pools are replaster jobs as a result of deterioration of the pool surfaces. Swimming pool plaster normally has a life expectancy of around 1 to 15 years depending on how it is taken care of chemically.
Swimming pool plaster is Portland cement blended with various types of crushed aggregate, and troweled to a smooth finish, normally 1/4" to 3/4" thickness. The plaster is generally acid sensitive resulting in etching, highlighting, aggressive nodules occurring on the plaster surface as a result of low water pH, low alkalinity, or aggressive water attacking the surface of the plaster.
Though deterioration of the plaster surface can occur at any age with an aggressive attack, highlighting or spot etching forms early in hydration when a susceptible cement allows penetration to the inner cement base surface. This type of etching is inversely proportional to the age of the plaster surface. After the first year, the plaster surface is much less susceptible to attack of aggressive water, e.g., than in the first month. A change in water chemistry from a stable to an aggressive is when a plaster surface can be permanently damaged. The longer a cement goes without an aggressive water attack, the less the damage when an aggressive water attack begins.
The pool plasters of the prior art absorb water. Water is a universal solvent. It creates both a corrosive and erosive environment for the pool plaster. The porous plaster surface absorbs surface stains that come from high calcium hardness which are difficult to remove. The weak points where the cement bonds to the aggregate are susceptible to aggressive water attack if the pH of the water in the pool falls too low. As a result, premature preferential etching of the surface can ruin a pool finish in less than a year by making it etched and highlighted.
In view of the problem of deterioration of the cement surfaces of swimming pools, it is desirable to provide an improvement in swimming pool cement plaster compositions which provides longevity of the plastered surface while continuously submerged under water, which typically has a low pH, and is very acidic and aggressive in nature, in relation to the surface of the cement plaster. Various attempts have been made to provide improved swimming pool cement plaster by modifying the plaster compositions. For example, acrylic polymers have been added to the cements to improve strength, durability and bonding. One such composition is disclosed in U.S. Pat. No. 4,430,463 to Mullenax but such additives require air-curing to reach full potential. Underwater cement curing and hydration is a must in the pool industry. Therefore silane-type materials become the ideal water repellency additive. For example, the prior art discloses a silicone treated lime product produced by Wacker Silicones, Adrian Michigan, which is used as an integral waterproofer for stucco. However, in the pool industry, additional lime in a plaster mix is not advisable. Lime is the product most likely to become soluble and to be leached.
It is therefore an object of the present invention to provide an improved cement pool plaster additive which extends the life of the plaster surface and thereby lowers the maintenance requirement of the plaster surface.
Another object of this invention is to provide a swimming pool cement plaster composition that can be cured underwater and which is virtually impervious to water during early stages of cement hydration.
Still another object of the present invention is to provide a process for coating the surface of a swimming pool with a cement plaster containing the additives of this invention.
These and other advantages of the present invention will become apparent to those skilled in the art from the following detailed description.